This invention relates to potentiometric position sensors.
Potentiometric position sensors find widespread application in industry in the measurement of angle or linear position. By fixing one part of a potentiometer to one component and another to another component, relative movement between the components causes a change in the electrical potential sensed by a contact wiper which moves along the track of resistive material. This change may be sensed by suitable circuitry, and used to provide an indication of angle or linear position.
Potentiometers have been manufactured for many decades as self-standing components, usually consisting of a casing within which is an insulating substrate on which a resistive track is present. A wiper is held resiliently against the track and the position of the wiper may be changed by rotation of a spindle or linear movement of a sliding member. Conventionally, the spindle or sliding member has to be attached to a component of which the position needs to be sensed, and the housing likewise attached to a suitable fixed mounting. This can be laborious, prone to error, and it uses a lot of parts.
In recent years, particularly in the automotive industry, where there is increasing use of potentiometric position sensors in a variety of applications within motor vehicles and their engines, an approach to reducing the component costs has resulted in the supply of subassemblies consisting of a potentiometer element comprising a resistive track printed on to an insulating substrate and a wiper contact, usually pre-assembled on an insulating base. The subassembly is then assembled into the particular automotive part concerned, for example a carburettor, by the manufacturer of that part.
In practice, this approach is unsatisfactory, as automotive parts manufacturers are not well equipped to ensure effective error-free installation of the potentiometer parts into the final item; in particular, the clean room conditions appropriate for potentiometer manufacture are not observed, and handling of sensitive components presents problems which require specialised equipment and procedures. The failure rate is unacceptably high, and it is difficult to determine whether failure has occurred because of a defect in the subassembly supplied, or because of problems in integrating it into the final item being manufactured.
It is the object of the invention to provide a new approach to potentiometric sensor design which avoids the disadvantages noted above, and to enable the supply of products to manufacturers which may be simply and easily handled and installed in any item in which it is desired to determine change in the angular position of two relatively movable parts.
According to a first feature of the present invention, there is provided a potentiometric position sensor subassembly which consists of an electrically insulating substrate having formed thereon at least one arcuate resistive track and a rotary member bearing at least one wiper member, the substrate and rotary member having mutually engageable clip means formed thereon and configured in such a way that when clipped together the wiper member contacts the track and the rotary member can be rotated about an axis substantially coincident with the centre about which the arcuate track extends.
Preferably, the clip means consists of a circular resilient skirt adapted to engage with a circular formation formed on or in a cylindrical surface. Most preferably, the skirt consists of a series of individually resilient elongate fingers each having at or near its free end a bead or groove, and the cylindrical surface has a corresponding groove or bead formed thereon into which the bead or groove on the fingers may resiliently engage. Such an arrangement enables the subassembly to be simply formed by clipping together the two members under axial force. The mutually engaging formations may be so configured that the members clip together easily, but cannot be easily pulled apart.
Such an arrangement provides protection to both wiper and track, and because the two members are firmly clipped together, they can be shipped as a unit without packaging or handling problems.
Preferably the substrate member comprises means for electrical connection to the track, for example contact terminals in the form of pins, tags or sleeves, configured to form a plug or socket for electrical connection with a complementary socket or plug, e.g. on a wiring harness or loom.
In a particularly preferred arrangement, the substrate member has a generally cylindrical body having on one end face the track(s) and having formed on its other end face a plug or socket incorporating connectors electrically connected to the track(s).
The rotary member preferably has on its face opposite to that bearing the wiper(s) means adapted to engage with one of the components the angular position of which it is desired to measure, for example a suitable socket.
When fitting a subassembly according to the present invention to an article where the relative angular position of two parts thereof is to be sensed, the subassembly does not have to be taken apart, but rather can simply be fitted into place by locating the rotary member in engagement with one component of the part, usually a spindle or shaft, and the substrate memner with respect to another component thereof, usually a housing. Thus the relative positioning of the two members of the subassembly is determined by the relative positioning of parts of the article in question. In order to ensure smooth operation, it is desirable that the two members of the subassembly are relatively loosely fitted on to another, i.e. the rotary member may move slightly in a plane perpendicular to its axis of rotation, so as to accommodate any slight dimensional variation in the articles to which the subassembly is to be fitted. In a particularly preferred embodiment, the rotary member and substrate mamber may have a pin and socket connection set on the axis of rotation of the rotary member, where the exterior surface of the pin includes a plurality of projections which wear differentially as the parts of the subassembly rotate one the subassembly is installed to compensate for any slight play or eccentricity in the relatively movable parts of the article. Alternatively or additionally, the part of the rotary member adapted to engage with part of the article, for example a socket, may, if desired, be mounted e.g. on a resilient set of spokes or so-called spider, the distal ends of which are fixed to the rotary member. Such an arrangement also allows a certain amount of axial play between the spindle or the like on the article and the rotary member when the subassembly has been installed.
The arcuate track(s) and other resistive portion(s) on the insulating substrate are preferably formed of a plastics composition formulated to have appropriate electrical resistance characteristics. Such compositions are often called xe2x80x9cconductive plastics inksxe2x80x9d because they may easily be applied to substrates by way of printing techniques, particularly screen printing. A major advantage of such materials in connection with the present invention is that they may also act as suitable bearing surfaces. Thus, in particular, the rotary member may be provided with a central bearing surface which is urged against a central area of such plastics film when the sensor is assembled and in use. This constitutes a further feature of the invention, and one which may be applied to a wide variety of potentiometer constructions and designs.
Thus, according generally to a further feature of the invention there is provided a potentiometer construction including an electrically insulative substrate on to which is printed at least one track of resistive material, and wherein the substrate carries in addition one or more areas of such material which, in the assembled potentiometer, act as one or more bearing surfaces.
Such an approach is of particular value in volume manufacture, as it enables bearing features to be provided at the same time as the resistive tracks are laid down, e.g. by screen printing, i.e. there is no need for a separate manufacturing step to provide a bearing surface.
In the potentiometer constructions with the clip-on rotary member, this approach is particularly useful as a central boss on the rotary member may be arranged to bear on a central area of bearing material, printed at the same time as the tracks, to definine accurately the distance between the face of the rotary member bearing the wiper(s) and the track(s) and accordingly assisting in defining precisely the force with which the resilient wiper member(s) press(es) against the arcuate track(s).
Potentiometric position sensors constructed in accordance with the present invention may be used in a wide variety of applications. They may be easily adapted to fit on appropriate spindles in existing machinery with the insulating substrate being then fixed with respect to the bearing of the spindle, preferably after rotating the spindle a few times to ensure that it and the rotary member can rotate freely with respect to the substrate member.
Although in many cases the sensor of the present invention is designed to be used to sense angular change over a substantial angular range, it should be appreciated that the sensor may be configured to operate over a very small range of angular motion if appropriate. In such a case, the rotary member and substrate member may be configured with only partial arcuate clipping means, i.e. not extending around a full circle.